Hybridization of short-range and long-range charge transfer excited states in multiple resonance emitter

Multiple resonance (MR) thermally activated delayed fluorescence emitters have been actively studied as pure blue dopants for organic light-emitting diodes (OLEDs) because of excellent color purity and high efficiency. However, the reported MR emitter, 2,5,13,16-tetra-tert-butylindolo[3,2,1-jk]indolo[1′,2′,3′:1,7]indolo[2,3-b]carbazole (tDIDCz) based on bis-fused indolocarbazole framework could not demonstrate efficient triplet-to-singlet spin crossover. In this work, we report two isomeric MR emitters designed to promote triplet exciton harvesting by reconstructing the electronic structure of tDIDCz. To manage excited states, strong electron donors were introduced at the 2,5-/1,6-position of tDIDCz. As a result, 2,5-positions managed tDIDCz shows long-range charge transfer characteristics while preserving the MR nature. Quantum chemical calculation demonstrates direct spin-orbit coupling by long-range charge transfer and spin-vibronic coupling assisted reverse intersystem crossing by short-range charge transfer simultaneously contribute to triplet-to-singlet spin crossover. Consequently, high performance blue OLED recorded a high external quantum efficiency of 30.8% at a color coordinate of (0.13, 0.13).

This manuscript is the extension work previously studied by Lee and co-workers reported in Small Science in 2020. In this contribution, the authors revealed the introduction of the tDPA units at the 1,6-or 2,5-position onto tDIDCz that could induce large discrepancy of the MOs distribution and photophysical properties. For instance, 2,5-tDPAtDIDCz of was found to possess a small gap between ∆EST and ∆ETT and hence more efficient direct spin-orbit coupling (long range CT) and spin-vibronic coupling assisted RISC processes (short range MR). In practice, OLED device based on the 2,5-tDPAtDIDCz demonstrated high external quantum efficiency of up to 30% and blue emission peaking at around 470 nm with FHWM of 38 nm.
In fact, the use of multiple charge-transfer excited state to induce efficient and stable thermally activated delayed fluorescence was first demonstrated by Adachi et al in 2018. A second type of electron-donating unit in a donor-acceptor system induces effective charge transfer and locally excited triplet states, resulting in an acceleration of the RISC process while maintaining high photoluminescence quantum yield (Adv. Sci., 2018, 4, eaao6910). Based on the similar approaches, TADF or MRTADF emitters bearing multiple donors for fine-tuning their excited states alinement to enhance the efficiency of the RISC process of up to 10 5 − 10 6 s −1 has been reported recently, including but not limited the following publications: J. Mater. Chem. C, 2023, 11, 4210;Angew. Chem. Int. Ed. 2022, 61, e202209984;Angew. Chem. Int. Ed. 2022, 61, e202201588. On the whole, the use of tDPA as an excited state manager to the MR type tDIDCz core for manipulating energy alinement of the excited states maybe not a new concept to the OLED communality. Meanwhiles,.
As donor materials such as carbazole, indolocarbazole or triphenylamine derivatives demonstrate the similar donating ability that could impose similar effect on this system, the authors also fail to point out the uniqueness of tDPA as an excited state manager. It is highly anticipated that the author could show the readers the reason behind the molecular design, in addition to the attachment of tDPA to different positions of molecule as illustrated in the manuscript. On the whole, I am afraid the novelty of this manuscript does not meet the standard of Nature Communication. In this regard, I have several concerns to raise as follow: 1. The excited state characters of S1, T1 and T3 should be shown in the figure 1, the relative energies of these states are clearly out of scale. Please update the figure.
2. In paragraph 14, it is not enough to explain the improved EQE besides outcoupling effect. Is there any possible evidence that prove the enhancement of EQE? For example, the PLQY in mBisPCz-O-BN thin film etc… Host polarity does affect 1 CT state of emitter and hence energy level splitting for TADF emission (Adv. Optical Mater. 2022, 10, 2101343). Table 2, what is the device result of 1,6-tDPAtDIDCz in mBisPCz-O-BN host? As suggested that SVC assisted RISC is disabled due to large difference of ΔEST and ΔETT, different host polarity could have improved such problem for efficient TADF emission by 1,6-tDPAtDIDCz.

In
4. The authors mentioned that the PLQYs of 2,5-tDPAtDIDCz and 1,6-tDPAtDIDCz were 88 and 92%, respectively, indicating efficient radiative transition assisted by the MR core structure.
What is the rationale or evidence of this statement? 5. Following the pervious question, the increased CT character in the 2,5-tDPAtDIDCz emitter through the tDPA excited state manager did not largely degrade the PLQY. However, I found that the PLQY of tDPA is 60% in the solution state. The authors should provide the PLQY of tDPA in 1,3-di(9H-carbazol-9-yl)benzene (mCP) : diphenyl(4-(triphenylsilyl)phenyl)phosphine oxide (TSPO1).
6. In the experimental section of device fabrication: PEDOT:PSS layer in device is produced by wet coating technique. Please specify such fabrication process as well.
7. The operational stability result of the OLED should be provided. 8. Some typing problems are found in the manuscripts, for examples, "Therefore, 2,5-tDPAtDIDCz may show TADF emission, while 2,5-tDPAtDIDCz may exhibit conventional fluorescence." Please check thoroughly.

Dear reviewers
We would like to express our gratitude to the reviewers who spent valuable time to evaluate our work titled "Hybridized Multiple Resonance and Charge Transfer Excited States Using an Excited State Manager for Efficient Reverse Intersystem Crossing and Narrow Emission". We have revised the manuscript according to your helpful comments. I have attached the revised manuscript along with the response letter.

Reviewer #1
Lee et al. synthesized a pair of tDPAtDIDCz isomers that contained a tDIDCz MR core and two tDPA donors and studied the photophysical and electroluminescent properties of these materials. It was found that 2, 5-and 1, 6-substitution of tDPA group in tDIDCz results in forming different degrees of CT character in excited state, which is very crucial to create efficient RISC for TADF. The 2, 5-substituted isomer exhibited a stronger CT character as supported by the photophysical and theoretical data. As a result, the emission of 2,5-tDPAtDIDCz was effectively shifted from violet region of tDIDCz to deep blue region and the singlet-triplet energy gap was dramatically reduced to 0.18 eV. The discoveries are indeed very interesting to the organic electroluminescence community. However, there are several fundamental issues that may affect the credibility of their discussion/interpretation. Therefore, I am unable to recommend the manuscript for publication in its current form.
1. The ΔEST of 2,5-tDPAtDIDCz is 0.18 eV. If the RISC does indeed happen, experimental evidence only suggested that RISC is between T1 and S1 of the MR-TADF motif. This means that Figure 2 is incorrect, particularly the level of theory required to obtain the correct ordering/energy gap of Sn and Tn resulting incorrect interpretation/discussion of the results.
The authors undertake a detailed discussion regarding the energy levels afforded by DFT calculations. Although DFT can be useful in qualitative discussions of the compounds (e.g. orbital localisations, etc), the discussion regarding singlet/triplet energy levels here is not convincing. It is known that TD-DFT can sometimes struggle to predict singlet/triplet energy levels in MR-TADF materials accurately (for an in depth discussion, see J. Chem. Theory Comput. 2022, 18, 8, 4903-4918). The authors should discuss their choice of functional/relative benchmarks in more detail if any quantitative conclusions are to be drawn from the reported energy levels.

Response
I appreciate your comments and understand your concerns. According to your comments, we conducted in-depth discussion and analysis of the issues regarding the computational analysis.
It is meaningful to consider wave function method (e.g. SCS-CC2, EOM-CCSD) based on the paper (J. Chem. Theory Comput. 2022, 18, 4903−4918). This claim is best illustrated by the spin density plots in Figure 3 of the revised paper. When an electron donor moiety is introduced into the MR core (e.g. DABNA) functioning as an electron acceptor, the TD(A)-B3LYP level analyzes the S1 excited state of MR-TADF by long-range charge transfer (LRCT). However, the actual S1 excited state of MR-TADF materials usually exhibits short-range charge transfer (SRCT). Through these examples, we believe that the methodology based on the wave function method shows high reliability for calculating boron type MR-TADF emitters.
Therefore, to verify the accuracy of the single point calculation performed at the MPM1B95/G(d,p) level in our manuscript, we additionally performed ab initio calculation using various methods: TD-DFT calculation at B3LYP, M06-2X, gap-tuned ωB97XD * /g(d,p) level, and wave function method performed at EOM-CCSD/cc-pVDZ level. The errors between the calculated excitation energies and the experimental values (singlet energy; ES, triplet energy; ET, singlet-triplet energy splitting; ΔEST) are summarized in the table below. a Range separation parameter, ω value was optimally tuned as 0.1052. b Range separation parameter, ω value was optimally tuned as 0.1030. Both parameters were obtained by following method: J. Chem. Theory Comput. 2015, 11, 3851−3858. A comparison between methods is as follows. First, the EOM-CCSD/cc-pVDZ level provided a modest error in estimating ET, but an abnormally large ES. For this reason, we excluded the results of the EOM-CCSD/cc-pVDZ level from the comparison between ab initio calculation methods. Overall, the B3LYP and MPW1B95 levels provided similar and significantly smaller errors in ES, ET and ΔEST. The gap-tuned ωB97XD * level showed high similarity with the experimental value for ET. However, it predicted large ES, which seems to be due to the underestimated long-range CT interaction. The M06-2X level provided a similar level of ΔEST to the gap-tuned ωB97XD * level, but both ES and ET showed large error. In summary, the results of this series of calculations provide a conclusion that the TD-DFT calculation at MPW1B95/G(d,p) level that we provided in the manuscript is not wrong, but rather provides a higher level of accuracy than other methods.
2. The authors claimed that SVC mechanism might facilitate RISC via high-lying triplet state.
Generally, different orbital type between singlet and triplet is the key to an efficient RISC (J. Am. Chem. Soc. 2017, 139, 4042). Looking at Figure 2, only a little change in orbital better S1 and T3 of 2,5-tDPAtDIDCz, which is less than that of 1,6-tDPAtDIDCz. These results are opposite to the trend of TADF efficiency of the two isomers.

Response
I appreciate your comments. The paper provided by the reviewer demonstrates the importance of the origin of singlet and triplet excited states to induce efficient RISC of TADF materials (specifically, D-A type TADF). In the D-A type emitters, the T1 state is required to possess locally excited (LE) character for efficient RISC by El-sayed rule because D-A type TADF materials clearly possess charge transfer (CT) character in S1 state. In line with this, the natural transition orbital (NTO) distributions of S1 and T3 states are consistent with the proposal of the previous paper, rationalizing the SVC-RISC mechanism.
The above figure shows the NTO distributions at S1 and T3 states that we provided in Figure   2, Figure S2 and Figure S3 (blue occupied: HONTO, red occupied: LUNTO). LUNTO is mainly distributed in DIDCz core that functions as an electron acceptor, while tDPA units contribute to the formation of HONTO. However, in the case of 1,6-tDPAtDIDCz, the DIDCz core also contributes to the HONTO, providing significant HONTO-LUNTO overlap. On the other hand, in the case of 2,5-tDPAtDIDCz, the contribution of DIDCz core to HONTO formation is negligible. From this, we can see that the S1 state of 2,5-tDPAtDIDCz can be regarded as CT excited state, and the S1 state of 1,6-tDPAtDIDCz can be regarded as a hybridized LE-CT excited state. In fact, photoluminescence and EL spectra revealed that a LE featured emission with a narrow FWHM is observed in 1,6-tDPAtDIDCz, and a relatively broad CT featured emission is observed in 2,5-tDPAtDIDCz (Figure 3 and Figure 5

(b)).
It is advantageous that the origin of excited state of the T3 state, which is the high lying Tn state that contributes to SVC-RISC, is the LE state judging from the localized HONTO and LUNTO on the DIDCz core. Therefore, the T3 state of the two emitters can mediate SVC-RISC.

Added sentence (1)
In addition, the T3 state of 2,5-tDPAtDIDCz had an excitation energy similar to that of the S1 state and it is predicted to be a LE dominant state because HONTO and LUNTO are distributed in DIDCz core. This implies that the close-lying T3 state is suitable as the SVC-mediating highlying Tn state for CT dominant S1 state to induce efficient RISC.

Added sentence (2)
In summary, 2,5-tDPAtDIDCz exhibited sufficiently small ΔEST due to the introduction of the excited state manager, and the reconstructed triplet excited states provided appropriate NTO distributions and SOCME values suitable for harvesting triplet exciton via direct SOC and 2 ndorder RISC. On the other hand, as can be seen in 1,6-tDPAtDIDCz, the linearly extended arrangement of two tDPA units could not build proper electronic structure suitable for triplet exciton harvesting.
3. There is no such thing called MR excited states. Short-range CT or locally excited state is a better description (Nat. Photonics 2021, 15, 780). So, "hybridization of the CT and MR excited states" is suggested to replace with hybridization of CT and LE excited state (HLCT) for the MR materials (Adv. Funct. Mater. 2023, 33, 2211893).

Response
I appreciate your comment. According to your comment, we changed article title and MR excited state to short range CT state.

Modified sentence
The excited state manager red-shifted the emission spectrum from violet to deep blue region and opened the RISC channel for TADF emission by hybridizing long-range CT (LRCT) and short-range CT (SRCT) excited state character.
It was demonstrated that direct RISC by LRCT and spin-vibronic coupling (SVC) assisted RISC by SRCT through MR core structure simultaneously contributed to the up-conversion of triplet excitons.
This work proposed that the hybridization of LRCT and SRCT excited states can realize both high EQE and narrow emission spectrum.
The hybridization concept of LRCT and SRCT excited states is to utilize the narrow emission from MR character and efficient RISC from CT character as presented in Figure 1.
Therefore, the S1 energy of 2,5-tDPAtDIDCz was significantly lowered by LRCT interaction between para-oriented sp 2 nitrogen of ICz and sp 3 nitrogen of tDPA.
The slightly broadened spectrum and weak shoulder in the 2,5-tDPAtDIDCz propose combined emissions from LRCT and SRCT excited states.
The participation of the rigid MR chromophore for fluorescence sharpened the emission spectrum while achieving high EQE by TADF through combined CT properties.
In this work, deep-blue MR type emitters were developed by managing the excited states of tDIDCz using a tDPA excited state manager which enables hybridized LRCT and SRCT excited state.
As a result of hybridization of LRCT and SRCT excited state, 4. In Figure 4a, 1,6-tDPAtDIDCz showed a biexponential transient PL delay profile with a long tail beyond 200 ns. Furthermore, the use of bicomponent mCP:TSPO1 host may have an exciplex character that exists RISC channel for TADF. The guest-host interaction should be considered.

Response
I appreciate your comment. In fact, mCP:TSPO1 is not an exciplex host. The results of the previously reported paper (Mater. Horiz., 2022, 9, 1299-1308) support this (please refer to Figure S10, solid PL spectra). Therefore, it is considered unlikely that mCP:TSPO1 host system activates additional RISC channel.
We reported that 1,6-tDPAtDIDCz is a deep blue fluorescent emitter with no TADF characteristics due to large ΔEST. To prove this, the exciton decay curve was recorded up to 1 ms time range. Consequently, no TADF characteristic was observed for this material.

Added and modified sentence
The tDIDCz emitter was a conventional fluorescence emitter without any TADF emission, but it was transformed into a TADF emitter by modifying 2 and 5-positions of tDIDCz with tDPA, whereas 1 and 6-position modification of tDIDCz with tDPA did not deliver TADF characteristic ( Figure S6).

Response
I appreciate your comment. We corrected the mistake. The corresponding sentence explained that the triplet energy of 2,5-tDPAtDIDCz was larger than that of 1,6-tDPAtDIDCz in terms of molecular structure. 1,6-tDPAtDIDCz has a chemical structure with two tDPA units aligned through para position of the aromatic unit in a linear fashion. Therefore, the conjugation is extended. In the case of 2,5-tDPAtDIDCz, the two tDPA units are attached to the meta position of the through the aromatic units in a bent fashion. Therefore, the conjugation extension is disrupted.

Reviewer #2
The authors developed a deep-blue MR type emitter by managing the excited states of tDIDCz using a tDPA excited state manager which enables hybridized CT and MR excited state. The device exhibited a high external quantum efficiency of 30.8% and a narrow emission with a full width at half maximum of 38 nm, and a color coordinate of (0.13, 0.13). This manuscript is suggested to be accepted after addressing the following issues: 1. The characterization of material photophysical properties and OLED are brief. The CIE coordinates, EQE-Luminance and PE-Luminance curve should be presented in the text.

Response
I appreciate your comment. We added EQE-L and PE-L curves to specify the device characteristics. In addition, the device data of the newly fabricated mBisPCz-O-BN:1,6-tDPAtDIDCz device was also added. Table 2 was modified to specify their device characteristics, CIE color coordinates and turn-on voltage (for response of comment 2). The added EQE-L and PE-L curves are illustrated in Figure S10 and Figure S11. Table 2. Summarized device performance of the blue OLEDs.

Compound
Von

Added sentence
The EQE-luminance and power efficiency-luminance curves of mCP:TSPO1 hosted devices and mBisPCz-O-BN hosted devices are illustrated in Figure S10 and Figure S11.
2. In the device performance table, the Von should also be listed.

Response
I appreciate your comment. We added turn-on voltage (Von) and driving voltage (Vd) of fabricated devices in modified Table 2. Please check the response of comment 1.

Response
I appreciate your comment. We developed two isomeric blue emitters, 2,5-tDPAtDIDCz and 1,6-tDPAtDIDCz, by introducing the tDPA unit into the MR core. Our motivation was to systematically study the effect of the donor substitution position on the emission properties of the MR emitters. Despite similar chemical structure, large differences of photophysical properties and device performances were observed.

Added sentence
Different effects of donor substitution position (2,5-or 1,6-position) on tDIDCz core provided formation of significantly different electronic structures.
4. The device fabrication procedure is too brief. Please add detailed information in the device fabrication section.

Response
I appreciate your comment. We added the information about detailed device fabrication procedure.

Reviewer #3
This manuscript is the extension work previously studied by Lee and co-workers reported in Small Science in 2020. In this contribution, the authors revealed the introduction of the tDPA units at the 1,6-or 2,5-position onto tDIDCz that could induce large discrepancy of the MOs distribution and photophysical properties. For instance, 2,5-tDPAtDIDCz of was found to possess a small gap between ΔEST and ΔETT and hence more efficient direct spin-orbit coupling (long range CT) and spin-vibronic coupling assisted RISC processes (short range MR).
In practice, OLED device based on the 2,5-tDPAtDIDCz demonstrated high external quantum efficiency of up to 30% and blue emission peaking at around 470 nm with FHWM of 38 nm.
In fact, the use of multiple charge-transfer excited state to induce efficient and stable thermally 1. The excited state characters of S1, T1 and T3 should be shown in the figure 1, the relative energies of these states are clearly out of scale. Please update the figure.

Response
I appreciate your comment. We modified figure 1 to clearly specify the exact excited state energy. The excitation energy was referred to calculated TD-DFT simulation data conducted at B3LYP/G(d) level.

Added sentence
To understand the origin of improved EQE in the mBisPCz-O-BN hosted device, transient PL and PLQY analysis of the emitter doped mBisPCz-O-BN films was conducted ( Figure S10).
The PL analysis revealed that mBisPCz-O-BN:2,5-tDPAtDIDCz film provided an improved PLQY of 92% and significantly lowered kISC while maintaining kRISC. Therefore, it can be concluded that the mBisPCz-O-BN host dramatically improved the EQE of the 2,5-tDPAtDIDCz device by enhancing the outcoupling efficiency and PLQY compared to the mCP:TSPO1 host.
3. In Table 2, what is the device result of 1,6-tDPAtDIDCz in mBisPCz-O-BN host? As suggested that SVC assisted RISC is disabled due to large difference of ΔEST and ΔETT, different host polarity could have improved such problem for efficient TADF emission by 1,6-tDPAtDIDCz.

Response
I appreciate your comment. We fabricated and evaluated a mBisPCz-O-BN hosted 1,6-tDPAtDIDCz device. The device characteristics of this newly fabricated device are provided in Figure S8. The recorded EQEMax was 7.3%, and there was no significant difference compared to that of the mCP:TSPO1 hosted device. In other words, 1,6-tDPAtDIDCz did not activate the SVC assisted RISC process even in the mBisPCz-O-BN host, and as a result, it showed only fluorescent emission without TADF. Figure S8. (a) The device structure and energy level diagram of mBisPCz-O-BN hosted blue OLEDs. (b) Normalized EL spectra, (c) current density-voltage-luminance curves, (d) external quantum efficiency-current density curves of 2,5-tDPAtDIDCz (blue) and 1,6-tDPAtDIDCz (black) devices.
Although the EQE was slightly improved, there was no significant difference compared to that of the mCP:TSPO1 hosted device. The host matrix did not activate the SVC-RISC process of 1,6-tDPAtDIDCz, and 1,6-tDPAtDIDCz device still showed only fluorescence without TADF process.
What is the rationale or evidence of this statement?

Response
I appreciate your comment. It was reported that the backbone structure of 2,5-tDPAtDIDCz and 1,6-tDPAtDIDCz, tDIDCz, exhibited PLQY of 60% (Small 2020, 16, 1907569). In general, it is known that when an auxochoromophore such as diphenylamine is introduced into the choromophore, it provides an additional n-π * transition to the chromophore, allowing a more efficient radiative transition. In this work, the PLQY was enhanced from 60% to about 90%. 5. Following the pervious question, the increased CT character in the 2,5-tDPAtDIDCz emitter through the tDPA excited state manager did not largely degrade the PLQY. However, I found that the PLQY of tDPA is 60% in the solution state. The authors should provide the PLQY of tDPA in 1,3-di(9H-carbazol-9-yl)benzene (mCP) : diphenyl(4-(triphenylsilyl)phenyl)phosphine oxide (TSPO1).

Response
I appreciate your comment. I cannot clearly understand the reviewer comment, but it seems that the reviewer points out how 2,5-tDPAtDIDCz can exhibit a high PLQY of 88%, even though tDPA, a fragment of 2,5-tDPAtDIDCz, exhibits a PLQY of 60%. The tDPA unit is just an auxochromophore that supports the MR chromophore, tDIDCz. This means that the origin of emission of 2,5-tDPAtDIDCz is not the tDPA unit. Please refer the following solution PL emission spectrum of diphenylamine (DPhA) reported in Talanta 121 (2014) 239-246.
Through this figure, we can confirm that the emission wavelength of diphenylamine is less than 400 nm and possess a large singlet energy. This fact suggests that even if a diphenylamine derivative such as tDPA is incorporated into the tDIDCz core, tDPA cannot be the major contributor of emission. Therefore, we believe that the PLQY data in mCP:TSPO1:tDPA doped film have no correlation with the photophysical properties of 2,5-tDPAtDIDCz and 1,6-tDPAtDIDCz.
6. In the experimental section of device fabrication: PEDOT:PSS layer in device is produced by wet coating technique. Please specify such fabrication process as well.

Response
I appreciate your comment. We have added a detailed explanation of device fabrication process.

Added sentences
A 40 nm thick PEDOT:PSS film was embeded on ITO substrate by spin-coating (30 s at 3,200 rpm) followed by annealing at 150 °C for 15 min.
7. The operational stability result of the OLED should be provided.

Response
I appreciate your comment. We added the device operational stability of blue MR-TADF OLEDs. The lifetime test was performed at a constant current density from an initial luminance of 100 cd m -2 until the luminance reached 50%. However, the lifetime was short due to instability of the host and charge transport materials.
Added supporting figure Figure S13. Luminance-lifetime curves of four MR-TADF OLEDs.
Added sentence Figure S13 provides the device stability measured under a constant current density condition at an initial luminance of 100 cd m -2 . All devices exhibited half device lifetime of less than 3 h, which presumed to be due to poor material stability of host and charge transport layer.

Response
I appreciate your comment. We checked the sentences and modified them in the main text